Gravity gradiometer

Abstract

A gravity gradiometer for measuring off-diagonal components of the gravitational gradient tensor includes a housing comprising a pair of electromagnetic shield enclosures (22, 23) arranged one inside the other, and a body (25) including superconducting material mounted within the inner enclosure (23) for fine pivotal flexure about an axis passing substantially through the center of mass of the aforesaid body. An array of superconducting coils (30) is supported by the outer enclosure (22) and positioned in close proximity to the aforesaid body (25) for diamagnetically applying a rotational force to the body with respect to the axis of flexure and/or for responding by modulation of inductance to pivotal flexure of the body arising from a gravitational gradient across the body. The array is arranged to apply the rotational force in both rotational directions and to respond to flexure in either rotational direction. Also disclosed is a flexural pivot bearing which comprises a pair of members (28, 31) with opposed close-spaced faces. These faces are joined by a web (29), of microscopic thickness, in a plane intersecting the faces. The members and the web are consisted of an integral body of substantially uniform, material, and the members are adapted for pivoted mutual flexure about a pivot axis aligned along said web.

We claim: 
     
       1. A gravity gradiometer comprising: a housing;   a body located within the housing;   a bearing for mounting the body for rotation about an axis passing substantially through the center of mass of the body, said bearing includes a web which undergoes limited elastic deformation in order to permit said rotation of the body about said axis; and   a plurality of transducers located within the housing for producing output signals in response to rotation of the body about said axis arising from a gravitational gradient across the body.   
     
     
       2. A gravity gradiometer as claimed in claim 1, wherein said plurality of transducers are arranged to apply rotational forces to said body. 
     
     
       3. A gravity gradiometer as claimed in claim 2, wherein said rotational forces operate to increase the effective stiffness of the web. 
     
     
       4. A gravity gradiometer as claimed in claim 1, 2 or 3, wherein said web is thin but is elongate in the direction of said axis. 
     
     
       5. A gravity gradiometer as claimed in claim 4, wherein said bearing comprises a pair of members and said web extends between said members. 
     
     
       6. A gravity gradiometer as claimed in claim 5, wherein the web is integral with at least one of said members. 
     
     
       7. A gravity gradiometer as claimed in claim 6, wherein the web is integral with both of said members. 
     
     
       8. A gravity gradiometer as claimed in claim 6, wherein said members are integral with said body. 
     
     
       9. A gravity gradiometer as claimed in claim 8, wherein the thickness of the web is less than the length of the net as measured in a direction which is perpendicular to said axis. 
     
     
       10. A gravity gradiometer as claimed in claim 9, wherein said web has generally planar opposed parallel faces symmetrically disposed relative to a perpendicular plane which includes said axis. 
     
     
       11. A gravity gradiometer as claimed in claim 10, wherein said body includes a cut which defines said members and said web and wherein one of the members comprises a core which is coupled to said housing and the other of the members comprises an adjacent face of a cavity formed in the body by said cut. 
     
     
       12. A gravity gradiometer as claimed in claim 11, wherein said core includes a part cylindrical surface and said cavity includes a portion thereof which is complementary thereto. 
     
     
       13. A gravity gradiometer as claimed in claim 12, wherein the spacing between said cylindrical surface and said portion is uniform. 
     
     
       14. A gravity gradiometer as claimed in claim 1, wherein said housing comprises an electromagnetic shield. 
     
     
       15. A gravity gradiometer as claimed in claim 14, wherein said transducers comprise superconducting coils. 
     
     
       16. A gravity gradiometer as claimed in claim 14, wherein said body comprises superconducting material. 
     
     
       17. A gravity gradiometer as claimed in claim 16, wherein said superconducting material is niobium. 
     
     
       18. An instrument comprising a pair of gravity gradiometers as claimed in claim 1 or 3, the housings of which are coupled together such that there is no relative movement therebetween. 
     
     
       19. An instrument as claimed in claim 18, wherein the axes of the gradiometers are coaxial and the bodies are mutually orthogonal. 
     
     
       20. An instrument as claimed in claim 19, wherein the housings are coupled to a gimballed suspension. 
     
     
       21. An instrument as claimed in claim 20, wherein the gimballed suspension is biaxial or triaxial. 
     
     
       22. An instrument as claimed in claim 21, including a container in which said suspension is located. 
     
     
       23. An instrument as claimed in claim 22, including a cryogenic fluid dewar in which said container is located. 
     
     
       24. A gravity gradiometer as claimed in claim 1, wherein the housing includes a pair of electromagnetic shield enclosures arranged one inside the other and said body includes superconducting material and is mounted within the inner of said enclosures and said transducers comprise an array of superconducting coils supported by the outer of said enclosures and positioned in close proximity to said body. 
     
     
       25. A gravity gradiometer according to claim 24, wherein said two enclosures are close fitting oblong boxes and said body is an oblong solid body of a shape complementary to said enclosures. 
     
     
       26. A gravity gradiometer according to claim 24, wherein an axis of flexure divides said body into respective arms of said body to either side of said axis of flexure, and wherein there is a superconducting coil on opposite sides of each arm. 
     
     
       27. A gravity gradiometer according to claim 26, wherein there are further coils to either side of the body at said axis of flexure and at each end of the body, for monitoring translational movement of said body. 
     
     
       28. A gravity gradiometer according to claim 24, wherein said superconducting material is niobium. 
     
     
       29. A gravity gradiometer according to claim 24, supported in a system which is shielded electrically, magnetically, thermally and vibrationally. 
     
     
       30. A gravity gradiometer according to claim 24, wherein said bearing is cut from a single mass of a superconducting material. 
     
     
       31. An instrument according to claim 26 comprising a pair of gravity gradiometers, wherein the housings are coupled together so that there is no relative movement therebetween and the axes of flexure of said respective bodies are substantially co-incident. 
     
     
       32. An instrument according to claim 31, wherein the arrays of superconducting coils associated with the two bodies are coupled into five superconducting loops and, in each loop, magnetic flux can be independently set and locked, which loops include a first loop including coil(s) at one or both ends of one of the bodies, a second loop including coil(s) at one or both ends of the other of the bodies, third and fourth loops including the coils to apply rotational force to and/or respond to pivotal flexure of the respective bodies, and a fifth loop responsive to the temperature about the bodies. 
     
     
       33. A gravity gradiometer as claimed in claim 5, wherein said pair of members and said web are integral with said body. 
     
     
       34. A gravity gradiometer as claimed in claim 33, wherein the amount of rotation of said bearing is limited. 
     
     
       35. A gravity gradiometer as claimed in claim 34, wherein the amount of rotation of said bearing is limited such that said web only undergoes elastic deformation. 
     
     
       36. A gravity gradiometer as claimed in claim 34, wherein the amount of rotation of said bearing is limited such that the bearing rotates through about 3°. 
     
     
       37. A gravity gradiometer as claimed in claim 33, wherein the web is about 0.03 mm thick. 
     
     
       38. A gravity gradiometer as claimed in claim 33, wherein the housing is coupled to a gimballed suspension. 
     
     
       39. A gravity gradiometer as claimed in claim 38, wherein the gimballed suspension is biaxial or triaxial. 
     
     
       40. A gravity gradiometer as claimed in claim 39, wherein the suspension is mounted in a container which can be immersed in a cryogenic medium. 
     
     
       41. A gravity gradiometer as claimed in claim 40, wherein said container comprises a vacuum can and said medium comprises liquid helium. 
     
     
       42. A gravity gradiometer as claimed in claim 41, including a dewar for containing said liquid helium and said dewar is capable of being mounted in an aircraft or other moving vehicle.